Network interface cards installed within computers, e.g., personal computers, enable them to communicate within a computer network, such as ETHERNET. Typically, the network interface cards (or "NIC's", as they are sometimes called) have serial interfaces for translating signals between codes used for communication within the computer and those used for inter-computer communication. The NIC's also contain transceivers to send and receive communication signals over the network. The signals from the transceivers typically are filtered to reduce noise prior to delivery to the network. A conventional filtering circuit found in NIC's has an output transformer for suppressing common-mode noise. The serial interfaces, transceivers and filter circuits are typically mounted on a NIC printed circuit ("pc") board.
A particular type of network interface card, called an ETHERNET adapter, is used in Europe to connect personal computers to an ETHERNET network via shielded twisted-pair cable. In such cable, two pairs of signal lines, one for transmission and one for reception, are surrounded by a conductive shield.
While known network communication interfaces, including the just described Ethernet adapter, generally perform satisfactorily, stringent standards have been promulgated in various countries to reduce electromagnetic emissions from such communication devices. For instance, in the Federal Republic of Germany, Deutcher Bundes Post Directive 1046/1984 and Limit Class B of DIN Verband Deutcher Elektrotechniker ("VDE") pertain to personal computers equipped with network interface cards. Computer manufacturers have had difficulty heretofore in meeting such standards, and in particular the just-mentioned German standards, using conventional techniques for reducing emissions.